Hello @dlafleur 
This is a challenging question. To start with, the first thing I think of is: which DNS server is your AWS scan server configured to use? 
I'm wondering if the AWS scan server is configured to use a DNS in AWS? You may want to try putting in a DNS server found in your on-prem environment instead. But this may also require configuring DNS traffic to come through your corporate and AWS firewalls (port 53). 
Let's confirm and try this first, run a scan, and see what results are returned. You might also want to test that the AWS scan server is able to reach and get results from the DNS server on-prem by using nslookup. (for example: nslookup (ip of target) (ip of dns server). 
Hope this gives us a little more clue(s) into what's happening. 
					
				
			
			
				
	Tim N.
Lansweeper Employee